Constitutively activated oncogenic mutants of the ABL1 tyrosine kinase play a central role in the pathogenesis of acute and chronic leukemias. Activation usually occurs as a consequence of chromosomal translocations (BCR- ABL1, TEL-ABL1 and others) or episomal amplification (NUP214-ABL1). Leukemias expressing oncogenic forms of the ABL1 kinase usually contain the non-mutated allele encoding normal ABL1 kinase which may play an important role in pathogenesis of disease and/or in response to treatment, given its prominent role in regulation of cell motility, adhesion, autophagy, response to DNA damage, apoptosis and proliferation. We recently reported that BCR-ABL1-Abl1-/- murine bone marrow cells generated highly aggressive chronic myeloid leukemia-blast phase (CML-BP)-like disease in mice compared to less malignant CML-chronic phase (CML-CP)-like disease from BCR-ABL1-Abl1+/+ cells. Additionally, loss of ABL1 stimulated proliferation and dramatic expansion of BCR-ABL1 murine leukemia stem cells, arrested myeloid differentiation, inhibited genotoxic stress-induced apoptosis, and facilitated accumulation of chromosomal aberrations. Moreover, we reported that in the absence of ABL1, BCR-ABL1 cells displayed reduced sensitivity to tyrosine kinase inhibitors (TKIs) such as imatinib; conversely allosteric stimulation of the ABL1 kinase enhanced anti-leukemia effect of TKIs in BCR-ABL1- positive CML and BCR-ABL1-positive acute lymphoblastic leukemia (ALL). We postulate that normal ABL1 is a tumor suppressor and therapeutic target in leukemias induced not only by BCR-ABL1, but also by other oncogenes. To test these hypotheses we propose 3 specific aims. Specific Aim #1. Identification of genetic aberrations benefiting from the loss of ABL1 to induce leukemia. a) We will determine the role of ABL1 loss in leukemogenesis induced by chromosomal translocations identified in the CGAP Mitelman database. b) We will perform genome-wide CRISPR screen to detect genetic aberrations promoting malignant transformation of Abl1-/- hematopoietic cells, but not their Abl1+/+ counterparts. Specific Aim #2. Mechanisms of ABL1-mediated tumor suppressor activity. a) We will employ ABL1 mutants to determine if nuclear and/or cytoplasmic ABL1 is a tumor suppressor. b) We will pinpoint upstream and downstream signaling pathways responsible for ABL1 tumor suppressor activities. Specific Aim #3. ABL1 kinase as therapeutic target in leukemias. a) We will determine if allosteric activator of ABL1 (DPH) also enhances anti-leukemia efficacy of TKIs targeting FLT3(ITD) and JAK2(V617F) in primary AMLs/MPNs cells. b) We will test novel therapeutic approach combining DPH + already approved drugs using primary leukemia xenografts carrying BCR-ABL1, FLT3(ITD) or JAK2(V617F). c) We will test more potent ABL1 kinase activators. This proposal may identify novel diagnostic marker (loss of ABL1) predisposing for malignant progression of leukemia. Moreover, ABL1, when expressed may be a valid target for novel anti-leukemia modalities.